It is well known that the speed of a separately excited DC motor is proportional to its supply voltage. In addition, speed is independent of load if the motor internal voltage drop due to the motor armature current is zero, or if the supply voltage to the motor is increased by an amount equal to the internal voltage drop.
The natural stabilization of the speed of rotation of such a motor to the supply voltage results in speed variations of a few percent, as the following example demonstrates. Assume that a separately excited DC motor which requires a current of 0.6 amperes to turn it has a back-electromotive force E of 12 volts, a hot resistance R of 1 ohm. Also assume that this motor is fitted with a capstan to unwind a tape, such as a magnetic tape, and that forces causing irregularities in the tape movement are .+-.50% of the nominal friction (these irregularities being reflected by a variation in current .DELTA.I of 0.3 A), the variations .DELTA.E in the back-emf which result in motor speed changes are thus equal to: ##EQU1## the stabilities of the back-emf and speed are therefore remarkable.
If high motor acceleration is required, the motor current is considerable and in practice depends only on the motor electrical characteristics and the inertia involved, with friction playing only a very small part. As a result, the servocontrol chain for this motor is of the analog type. The chain reacts to a speed indicating signal derived as the motor back-emf, as derived from the motor winding. The chain is made up, in essence, of a power amplifier supplying the motor armature, an analog regulator, and a generator for a control signal that indicates the speed required by an operator. The required speed signal is applied to the analog regulator to energize the power amplifier and operate the motor accordingly.
Such a servocontrol chain is advantageous because with the speed "sensor" being the motor winding, there is no delay in receiving signals for the motor speed. The chain is very simple, well known to those skilled in the art, highly reliable, and naturally stable. It does not require phase-correcting circuits and, as demonstrated above, constant speed is obtained.
Despite all these satisfactory qualities, the degree of stabilization achieved still falls short of that which may be desired in certain cases, because there is only partial compensation for the internal drop in the armature and due to differences in the back emf constant of different motors and as a function of temperature. A solution to the deficiency in performance caused by the partial compensation for the internal drop is found in my co-pending commonly assigned U.S. patent application Ser. No. 914,987 entitled "A system for Controlling a Separately Excited, Constant Load DC Electric Motor" filed June 12, 1978; this solution is used in the described embodiment of a servocontrol system according to the present invention. Other causes of deficiency mentioned above are random variables: (1) inherent in the manufacture of the motors and ancillary components of the motors, and (2) in the conditions in which the motors are used (ambient temperature for example). Hence, it is necessary to have recourse to another method of servocontrol to provide for variables of this nature.
The search for a servocontrol capable of bringing the motor accurately to a required position so that a driven member, such as a magnetic tape, assumes a required position, has usually involved servocontrolling the speed of the motor with analog or digital techniques.
Analog techniques seem to be necessary for improving the simple analog servocontrol chain described above if all the advantages of the chain are to be effectively preserved. However, because of the nature of the causes of deficiency described above, the system becomes very complicated and the required adjustments militate against mass production and easy trouble shooting, causing relatively high costs.
Digital techniques have the advantages of providing the expected results and of avoiding adjustments. Nevertheless, if digital techniques are used, rather than the simple analog servocontrol system, very valuable advantage is sacrificed. In view of the very severe demands on the new servocontrol, the digital techniques require a large number of components to be used and complicate the circuitry.
At the present time, neither the analog nor digital methods of servocontrol provides the exact stabilization necessary to enable the motor driven member to be controlled to the precision required by an operator. However, in many systems, e.g. for driving magnetic tapes, increasingly exacting demands prevent the desired standards of performance from being achieved by currently envisaged servocontrol systems.